This invention relates to a hydraulic fluid power linear actuator employing a cylinder and combined piston and piston-rod assembly of elements to convert a supply of pressurized hydraulic fluid to linear mechanical force or motion.
Hydraulic fluid powered cylinders are used in a multiplicity of applications in many diverse fields such as aerospace, marine, military, industrial, automotive, and manufacturing. Most hydraulic fluid powered cylinders employed in these fields consist of devices where the extension rate of a given cylinder system is not a function of the load force requirements, but a function of the rate at which fluid is supplied to the cylinder. The available force at which a cylinder extends is usually a function of the characteristics of the load a cylinder must move; i.e., for a given cylinder bore, the greater the load, the higher the force at which the cylinder must operate. If it is desired to maintain a lower pressure, a larger bore cylinder would be required to move the same load.
Manufacturing operations such as clamping, pressing, die stamping and compacting of materials for bailing may be accomplished by such cylinders. Some of these types of operations would be improved greatly if, in the interest of saving time, the cylinder would extend at a rate that was inversely proportional to the load applied thereto. In other words, improved results would be provided if the actuator could supply power to the load at a more uniform rate. Since power is the product of rate of travel times force, this means providing a relatively fast extension rate when the load is light and a relatively slow extension rate when the load is heavier.
A conventional hydraulic fluid power cylinder has two inherent performance characteristics: (1) an increase in the rate of supply of hydraulic fluid to the expanding chamber of the cylinder is required for a proportional increase in the rate of motion of the piston rod and (2) an increase in the pressure differential across the piston is required for a proportional increase in the force that the piston can apply to a load.
Most liquid power linear actuators consist of devices where the extension rate of the piston and piston-rod combination is proportional to the rate at which the pressurized liquid is supplied. Thus, if pressurized liquid is supplied at a constant rate, such an actuator would extend at the same rate throughout its travel regardless of the load. Accordingly, these systems must be designed to meet maximum load conditions regardless of the relatively short intervals of time during which maximum load is applied to the piston rod. This can, and does, result in the poor and inefficient utilization of the pressurized liquid when the load is low and in an expensive waste of equipment, time, liquid and energy.
There are numerous applications wherein it would be desirable to have a cylinder that would travel at various speeds and exert varying forces upon varying loads. In the operation of a conventional cylinder at the beginning of a stroke, the cylinder will have to overcome a relatively small load (in most cases no more than internal friction) and can thus travel quite rapidly with the available power from the fluid source. During this portion of the cycle, the cylinder operates at low system pressure. However, the rate of speed at which the piston travels is controlled, not by pressure, but by fluid delivery rate. When the piston rod encounters a greater resistance (a larger load), in order to continue traveling at the same rate of speed, delivery rate remains constant while pressure increases. If a still greater load is encountered, but a relatively high speed is not required, as in the terminal portion of the stroke, delivery rate can be reduced and fluid delivery pressure increased.
To meet and overcome these and many other varying load conditions, various complex means have been devised and arranged to solve this problem.
Some of these complex systems utilize complex supply systems which provide more pressurized fluid when the load is low, resulting in a faster piston rod extension rate and then less liquid at a higher available pressure when the load is higher, resulting in a slower extension rate. Examples of such systems include variable displacement pumps, dual pressure liquid supply systems and systems which store hydraulic fluid under pressure, such as with accumulators.
Other liquid power linear actuators extend at a faster or slower speed, depending on load, while being supplied with pressurized liquid at a constant rate. A number of such systems involve complex combinations employing a plurality of piston and piston-rod cylinder assemblies. Others utilize a regenerative cylinder circuit which supplies additional pressurized liquid to the expanding back end chamber of the cylinder by feeding the pressurized liquid to the back end chamber from the rod end chamber.
Most of these regenerative cylinder circuits locate the necessary valving and liquid conduits outside of the cylinder, which usually results in a difficult and awkward arrangement because of design problems involving short liquid connections and because of the requirements for both high pressure and high flow conditions.
A few regenerative cylinder circuits locate the valving and liquid passages within the piston inside the cylinder. Generally, these circuits have found limited acceptance because of the manner in which four fundamental requirements are handled: (1) the need for controls which detect when the piston valve should be open or closed in the course of the work cycle, (2) the means by which this information is communicated to and acts on the piston valve, (3) the need to save time during all nonproductive time periods during a complete work cycle, with special attention to the return of the piston to its retracted starting position, and (4) the complexity of the device.
My invention provides an improvement upon the invention described in my co-pending application, Ser. No. 841,217, filed Oct. 11, 1977. The prior application utilizes a regenerative cylinder circuit with valving and liquid passages located within the piston inside the chamber. However, that invention provides a piston valve of different configuration from that of the present invention in that the valve disc is fixed to a valve stem requiring the two elements to cooperate in unitary movement in contrast to the configuration of the present invention which discloses independently movable discrete unattached valve disc and plunger. While my previous invention is a meritorious invention and is useful in practice, it has certain shortcomings. In my prior invention, the rapid advance mode is terminated by physical constants, namely the elastic characteristics of the valve spring and the cross-section area of the valve stem. In contrast, the termination of the rapid extension mode of the present invention is not dependent on such characteristics and will function with a wide range of geometrical relationships of component elements. In the prior invention, for a given cylinder assembly, changing the predetermined load value, which determines the end of the rapid extension mode, can only be accomplished by disassembling and modifying the piston valve assembly. Further, setting a high predetermined load value is difficult because of the complications presented by the physical requirements that call for a bias spring with a higher strength or a valve stem with a smaller cross-sectional area. Also, utilizing a relatively low predetermined load value for a particular work cycle will result in poor operating efficiencies since the higher available force in the rapid extension mode cannot be utilized. Lastly, utilizing a relatively high predetermined load value for a particular work cycle will result in better efficiency, but a more precise control of the external valving will be required to insure that the liquid is not allowed to exit from the rod end chamber below the predetermined load value, thereby rendering the system inoperative.
As a result of this different and improved valve configuration, hereinafter described in detail, the cylinder of the present invention extends at two speeds, and in three force modes. The cylinder of my prior invention extends at two speeds, but in only two force modes. As a result of the provision of an additional force mode substantial savings in the operating cycle of my cylinder is achieved, with consequent reduced cycle time and increased efficiency of operation.